When burning fuels like e.g. straw, wood and household refuse, the combustion process comprises the liberation of corrosive substances like sulphur, chlorine and alkali compounds. If the combustion occurs in connection with processes, in which the heat is utilized for producing steam, and in which the steam is subsequently to be superheated, e.g. for use in a steam turbine, the life span of the superheater depends in part on the content and concentration of corrosive substances in the flue gas, in part on the material, of which the superheater is manufactured, and the temperature of the material.
In a steam-producing plant, the superheater is normally an integral part of the total layout of the plant. The steam to be superheated flows under pressure in tubes made from metallic materials. The tubes are heated externally by the hot flue gases from the combustion process, and if the steam is to be used e.g. in a steam turbine for producing electric power, it is of decisive importance for the performance of the turbine that temperature and pressure are as high as possible, i.e. as high as permitted by the materials used.
With the methods known at present for using flue gas from chlorine-containing fuels, it is not possible to achieve steam data as good as the data achieved with chlorine-free fuels, without the process causing high-temperature corrosion, rapidly causing breakdown of the materials, of which the superheater is constructed. The literature in this field describes materials able to resist the aggressive attack from e.g. chlorine and alkali compounds, even at high temperatures. These materials are, however, extremely costly and can hardly be used at the high pressures occurring in a superheater.
It is known to be possible, by using several fuels in combination, to achieve improved steam data compared to what is possible when using one single problematic fuel. This is done by using the problematic fuel to produce steam at low temperatures, while the superheating towards higher temperatures takes place in an external superheater, in which the high temperature is produced by burning a problem-free fuel, such as natural gas.
Further, the literature describes the possibility of achieving superheating by means of pyrolysis gas produced from the problematic fuel at low temperatures, so that the corrosive substances, such as e.g. chlorine and alkali compounds, are retained in the pyrolysis equipment. This process step is, however, extremely complicated and expensive, and is hardly likely to find widespread use in commercial plants.
The document U.S. Pat. No. 4,099,382 discloses a method and a plant for producing superheated steam utilizing the heat from a partial oxidation process. The method concerned features the same steps as are set forth in the preamble of claim 1, and could be contemplated as being useful for burning fuels of the kind referred to above, i.e. straw, wood and household refuse, while avoiding the disadvantages of the other methods referred to above with regard to avoiding corrosion at a reasonable cost.
In the method of said U.S. Pat. No. 4,099,382, the hot flue gas flowing from the combustion site, in this particular case a gas generator producing heat by partial oxidation, flows firstly to a heat exchanger transferring heat to the superheater, upon leaving the heat exchanger flowing to a steam-generating gas cooler (i.e. a boiler) giving off the remainder of its heat to the latter.
This arrangement makes it difficult to control the transfer of heat from the combustion site to the steam generator and the superheater independently of each other, because the heat exchanger and the steam generator are virtually "series-connected" with regard to the flow of hot flue gas from the combustion site. When burning fuels of the kind referred to initially, that may be delivered to an incinerating plant in highly varying proportions of the various ingredients, it is highly important to be able to control the two heat flows independently of each other in order to optimalize the combustion process at all times.